# Keys

A key is an attribute of the relation. There are several types of keys:

* **superkey**
  * K is a superkey of R if values of K are sufficient to identify a unique tuple of each possible relation r(R)
  * Ex. {ID} and {ID,name} are both superkeys of instructor
* **candidate key**
  * A candidate key is a minimal superkey
  * Ex. {ID} is a candidate key for instructor
* **primary key**
  * One of the candidate keys is selected as a primary key
  * A primary key must uniquely identify a tuple
* **foreign key**
  * A relation R1 may include an attribute that is the primary key of another relation R2; this attribute is called the foreign key from R1 to R2
  * R1 is called the **referencing relation** and R2 is called the **referenced relation**
  * The **foreign key constraint** (or **referential integrity constraint**) requires that the values of the attribute in the referencing relation must appear in at least 1 tuple of the referenced relation

In the university database figure (see previous section), the candidate keys for each relation have been underlined and the arrows denote foreign key dependencies.